1. Field of the Invention
The present invention is directed to a preferably ultra-selective high-frequency surface-active wave filter of that type that is referred to as dual mode surface-active wave (SAW) filter (DMS filter); this is also called a longitudinal mode resonator filter. These surface-active wave filters relate to electromechanical filters whose structure elements, such as transducers and resonators, are positioned on the surface of a piezoelectric substrate.
2. Description of the Related Art
The previously mentioned filters are known, for example, as one-track filters. For higher selection, filters are also produced and employed in which two such filter tracks are cascaded in a combined fashion into one filter and are arranged on a respective substrate. FIGS. 13A and 13B show a known single-track DMS filter and a known, cascaded two-track filter, the latter being composed of two one-track DMS filters interconnected with one another.
With reference to the respective surface-active wave track, these embodiments have respective end-residing resonator/reflector structures and respectively at least one interdigital structure as a transducer between these for signal input and for signal output.
The single track filter 10 shown in FIG. 13A comprises, a (first) transducer 1, two further (second) transducers 21, 22, and reflector structures 31. Here, the two transducers 21 and 22 are connected electrically in parallel with their alignment 5 as an input of the filter 10 for the surface wave 4 to be generated and used in the filter 10. As also indicated in FIG. 13A, these input transducers can be operated symmetrically or asymmetrically, with either a symmetrical signal input at both sides (IN bal/IN bal) or with an asymmetrical (IN unbal) input at one side to ground (IN ground). The terminals of the first transducer 1 employed as an output in this illustration are symmetrical outputs (OUT bal and OUT bal). The input and output can be interchanged in such a filter or can be employed in an interchanged manner.
FIG. 13B shows a known cascaded filter that comprises two tracks or, respectively, one-track filters 10, 110 connected to one another as shown. Reference characters of FIG. 13A are also employed here. Given this cascaded filter, for example, the transducer 1 is provided as an optionally asymmetrical/symmetrical input of the filter. The output of the filter is the transducer 1xe2x80x2. The other transducers 21, 121, 22, 122, as can be seen from the interconnection, are coupling transducers with which the two tracks 10 and 110 are electrically coupled to one another.
It is standard practice that the transducer 1 of the one-track arrangement according to FIG. 13A and the transducers 1 and 1xe2x80x2 of FIG. 13B are always implemented in a mirror-symmetrical manner with reference to the center plane M perpendicular to the alignment 5 of the surface-active wave and, accordingly, have an uneven plurality of inter-engaging fingers. These are respectively five interdigitally arranged fingers of the transducers 1 and 1xe2x80x2 in FIGS. 13A and 13B.
An object of the present invention is to provide an effective symmetry of the output signal of an appertaining filter given an asymmetrical or symmetrical input signal.
This object is achieved by a dualmode surface acoustic wave filter comprising an input terminal and an output terminal, at least one of the input terminal and the output terminal being balanced; a first filter track and a second filter track, each track comprising: at least one first transducer comprising an even-numbered plurality of transducer fingers electrically connected to a respective busbar and arranged interdigitally; a plurality of second transducers, each second transducer comprising an even-numbered plurality of transducer fingers electrically connected to a respective busbar and arranged interdigitally; and reflector structures; wherein: the second transducers serve as coupling transducers, the second transducers of the first filter track being electrically connected to respective second transducers of the second filter track, and the at least one first transducer of the first track serving as an input transducer; the at least one first transducer of the second filter track serving as an output transducer; at least one balanced terminal chosen from the input and output terminal being electrically connected to the busbars of the respective input and output transducer; in each filter track, a finger arrangement of the first transducer being point-symmetrical with respect to a center of the respective transducer; in each filter track, the at least one first transducer being arranged between two respective second transducers; in a respective track, a finger arrangement of the two second transducers being point-symmetrical to one another with respect to a center of the corresponding first transducer; or in a respective track, a finger arrangement of the two second transducers being mirror-symmetrical to one another with respect to an axis that is orthogonal to a wave propagation direction, which axis passes through the center of the corresponding first transducer.